1. Field of the Invention
The present invention relates to a process and to a device for measuring the velocity and the flow rate of a fluid stream.
2. Description of the Prior Art
Accurate determination of the circulation rate of fluids in pipes and of the corresponding flow rates is important in many fields, notably in chemical plants, chromatography, etc.
A known process for determining the velocity and the flow rate of a fluid stream circulating in a pipe is for example described in WO-93/14,382 or U.S. Pat. No. 4,308,754. It essentially consists in measuring the difference between the respective traveltimes of acoustic pulses between emitting and receiving transducers situated along a fluid feeder, at a known distance from one another, according to whether the waves are propagated upstream or downstream in relation to the direction of flow.
The flowsheet of FIG. 1 shows two piezoelectric type emitting-receiving transducers for example, arranged on either side of a pipe in which a fluid circulates at a velocity v, in two transverse planes thereof at a distance from one another. They simultaneously emit, one in the direction of the other (slantwise), ultrasonic pulses of frequency f0 (transducers tuning frequency) and of duration t0 much shorter than the traveltime of the waves between the two transducers. The arrival times tAB and tBA of the signals are measured and the acoustic transit times (or traveltimes) tv1 (in the direction of flow) and tv2 (in the opposite direction) are deduced by subtracting therefrom the different parasitic lag times obtained by calibration.
Propagation times tv1 and tv2 are respectively written as follows:       tv    1    =                    L                  C          +                      V            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢            α                              ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢              tv        2              =          L              C        -                  V          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          α                    
It is readily deduced therefrom that:   V  =                    L        ·        Δ            ⁢              xe2x80x83            ⁢      t                      2        ·                  tv          1                ·                  tv          2                ·        cos            ⁢              xe2x80x83            ⁢      α      
where xcex94t=tv2xe2x88x92tv1.
The flow rate is then expressed by Qv=v.S, if S represents the cross-section of the stream.
In a practical example where the transducers are about 10 cm apart and the celerity of the waves in the fluid is 1500 m/s, the traveltime is about 60 xcexcs. It can be noticed, with such a practical example, that if the desired accuracy is of the order of 10xe2x88x923 when measuring the velocity of flow, it must be possible to measure time intervals of the order of a few ns. This is very difficult to achieve by direct measurement of the propagation times with detection of the times when the energy received exceeds a certain threshold, because the accuracy is generally insufficient and implies working out many averages.
The process according to the invention notably overcomes this drawback and obtains, at a comparatively much lower cost than with the previous solution, a very high accuracy when measuring the displacement velocity of a fluid stream and consequently the flow rate of this stream.
The process of the invention allows determination of the velocity of flow of a fluid stream by comparison of the respective traveltimes of acoustic pulses respectively emitted and received between points spaced out along the fluid stream, according to whether they are propagated upstream or downstream in relation to the direction of flow. It is characterized in that the average traveltime and the difference between the traveltimes are measured by determination of the frequency spectrum associated with each pulse received and precise measurement of the relative phase lags affecting the frequency spectra of the acoustic pulses received, resulting from their traveltime.
According to an advantageous embodiment (suitable for relatively less absorbent fluids), a first acoustic pulse is emitted at each point, a second acoustic pulse is emitted from another point and an echo, at this other point, of the first acoustic pulse are successively detected at each point, the frequency spectra of the various pulses detected are calculated, and the average traveltime of the acoustic pulses detected and the differences between their respective traveltimes are determined.
According to another embodiment, the average traveltime is determined from reference spectra obtained by calibration from spectra of received acoustic pulses.
According to another embodiment, the difference between the respective traveltimes of the acoustic pulses received is determined from their frequency spectra and from a time difference obtained by calibration.
According to a preferred embodiment, the process comprises transmitting acoustic pulses simultaneously from a first point along a fluid stream in the direction of a second point downstream from the first point and vice versa from the second point in the direction of the first point, and detecting the pulses received at both points in fixed reception windows subjected to the same time lag in relation to the common times of emission of these pulses, the phase lag measured for each frequency spectrum depending on the position of the corresponding pulse received in the corresponding reception window.
To remove any ambiguity about the phase lag value, the slope of the line representative of the phase variation as a function of the traveltime is preferably determined on a determined portion of the frequency spectrum of the pulses.
A device allowing implementation of the method comprises for example at least two emitting-receiving transducers arranged in distinct places along a fluid stream, an impulse generator connected to the transducers, a signal acquisition unit samples and digitizes the signals received by the transducers during a fixed acquisition window and a processing unit for determining the phase lags affecting at least a portion of the frequency spectrum of each pulse received, due to the variable traveltime of the acoustic pulses emitted.
The processing unit comprises for example a signal processor programmed to determine the FFT frequency spectrum of each signal from a series of samples acquired in said window.
The process according to the invention provides very high accuracy for measurement of the traveltime of waves through the fluid in motion. It allows very short time intervals difficult to measure with accuracy under acceptable economic conditions to be translated into large phase variations with great amplification. Simulations showed that accuracies higher than 1‰ can be obtained for velocity measurement.